Magnetic tape provides a means for physically storing data. As an archival medium, tape often comprises the only copy of the data. The tape is typically made as thin as practically possible to maximize the length of a tape stored on a tape reel, and thereby maximize the amount of data that can be stored on the tape contained in a single cartridge. A tape drive is used to store and retrieve data with respect to the magnetic tape. Thinner tapes are desired to maximize the amount of tape stored in a single cartridge; however, thinner tapes are also more prone to breakage as a result of mechanical stress or malfunctions of the tape drive. An example of a tape drive is the IBM TotalStorage Enterprise Tape Drive 3592 manufactured by IBM Corporation. Tape drives are typically used in combination with an automated data storage library. For example the IBM TotalStorage Enterprise Tape Library 3494 manufactured by IBM Corporation is an automated data storage library that may include one or more tape drives and data storage media for storing data with respect to the tape drives.
Tape drives frequently employ DC motors and feedback control systems with motor drivers for operating the DC motors, to produce well controlled motion parameters such as position, velocity, and tape tension. Such control systems are usually very complex, and the feedback control system may compensate for marginal components, hiding latent problems until a catastrophic failure occurs. Tape breakage may occur as a result of a catastrophic failure or as a result of prolonged continuous use. If tape breakage occurs and the tape reels don't immediately cease rotation, further damage to the magnetic tape and/or tape drive may result. For example, rotation of the tape reels after tape breakage may result in the magnetic tape unwinding inside the tape drive resulting in severe damage to the tape and possible contamination of the tape drive from magnetic tape particles emitted from the disintegrating tape.
Tape breakage sensing has been accomplished previously by optical sensing systems that monitor the tape position on the guiding roller. A deviation from the expected range of motion indicates tape failure. Other systems use tension sensors coupled to the tape reel drive system to monitor the tape tension between the two reels. These solutions require additional sensors and associated circuits that are expensive and add to the cost of the tape drive.
Another problem with detecting tape breakage is that the tape breakage detection system may be prevented from operating when the tape drive experiences a condition that results in communication problems internally between the drive software, the servo software and/or hardware components. A typical tape drive system is controlled by microcode running on central processing unit (CPU) and/or application specific integrated circuits (ASIC). Multiple microcode sections may be responsible for the detection of abnormal conditions, and these microcode sections may experience communication problems during an error condition. The drive error detection processes depend on the drive behavior and the microcode status during the time when the error occurs. The error detection system may be required to examine many microcode variables depending on the code status and drive behavior when the error occurs. This may take a considerable amount of time during which the tape is further damaged. During certain conditions the system may cease to change states resulting in a locked or freeze condition. This results in a fatal error, with a possible power on reset being the only method to bring the drive back to operation. Severe damage may occur to the tape and/or the drive during a freeze condition. A periodic timer may be used to interrupt the drive and prevent the lockup condition. The periodic timer is used to periodically examine the CPU to ensure that the CPU continues to service the tape transport control system that controls tape movement. If an error occurs then the periodic timer will signal the CPU to disable the tape reel drive motors to stop the tape motion. The periodic timer is not always effective because it is possible for a tape drive system malfunction to occur that is not detected by the tape transport control system operating within the CPU. For example, the tape may run out of control while the servo control system appears to be operating correctly. In this case the periodic timer is not effective because the CPU does not detect the malfunction. Therefore, there is a need to improve the detection and compensation for magnetic tape breakage in a tape drive.